Energy robust field effect transistor

ABSTRACT

A field effect transistor ( 30 ) has an array of transistors ( 31 ) made up of bonding pads ( 45 - 47 ) and sub-arrays of transistors ( 41 - 43 ). The bonding pads ( 45 - 47 ) are distributed between the sub-arrays of transistors ( 41 - 43 ) to reduce the maximum temperature that any portion of the FET ( 30 ) is exposed to while the FET ( 30 ) is in a conducting state. A similar effect can be appreciated by adjusting the threshold voltage or pinch-off resistance of the transistors in a portion ( 101 ) of an array of transistors ( 95 ) or by providing transistors with a higher thermal breakdown in the center of the field effect transistor ( 30 ).

RELATED U.S. APPLICATION DATA

[0001] This application is a continuation in part of Ser. No. 09/562,604, May 1, 2000, which is a divisional of Ser. No. 09/088,027, Jun. 1, 1998, U.S. Pat. No. 6,140,184.

BACKGROUND OF THE INVENTION

[0002] This invention relates, in general, to semiconductor devices, and more particularly, to field effect transistors that are arranged in an array configuration for high power applications.

[0003] During operation, power FETs generate a significant amount of heat due to the relatively high power densities that dissipate through the FETs. The amount of heat that is generated can be enough to damage structures within the array, or at the very least, affect the mean time to failure of the device in a customer's application.

[0004] One technique to address this problem is shown in U.S. Pat. No. 5,003,370, which issued to Shunji Kashwagi on Mar. 26, 1992. The solution shown is directed to applications that operate at high frequencies (e.g., 900 MHz), and thus, applications with relatively short “on” times (approximately 1-50 nanoseconds). Simply stated, the solution involves increasing the effective distance between individual cells by configuring the cells in a zigzag pattern so that there is an inactive heatsink between each cell. However, the solution is only practical for devices that have short “on” times (less than 50 nanoseconds) because the heatsinks between each cell can only dissipate a relatively small amount of energy.

[0005] Accordingly, a need exists to provide a semiconductor device that has improved energy dissipation capability when the device is operated for a long period of time. It would also be advantageous if the device could be formed without increasing its size so as not to increase its manufacturing complexity or cost.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a block representation of a previously known power device;

[0007]FIG. 2 is a graph of the increase in temperature across the power device during operation;

[0008]FIG. 3 is a simplified representation of a field effect transistor in accordance with an embodiment of the present invention;

[0009]FIG. 4 is a top view of a portion of the field effect transistor in accordance with the present invention;

[0010]FIG. 5 is a top view of a portion of a sub-array of transistors of the field effect transistor in accordance with an embodiment of the present invention;

[0011]FIG. 6 is a cross-sectional view of the sub-array of transistors in accordance with an embodiment of the present invention;

[0012]FIG. 7 is a graph of illustrating the improvement in energy dissipation of an embodiment of the present invention;

[0013]FIG. 8 is a cross-sectional view of a semiconductor device in accordance with another embodiment of the present invention;

[0014]FIG. 9 is a top view of a portion of a field effect transistor in accordance with an alternative embodiment of the present invention;

[0015]FIG. 10 is a top view of a portion of a field effect transistor in accordance with yet an another embodiment of the present invention;

[0016]FIG. 11 is a cross-sectional view of the field effect transistor in accordance with the embodiment illustrated in FIG. 10; and

[0017]FIG. 12 is a graph illustrating the improvement in energy dissipation of another embodiment of the present invention.

[0018] It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE DRAWINGS

[0019] Power transistors are typically arranged as an array of repeating cells with each of the cells providing a portion of the current for the power transistor. For example, FIG. 1 is provided only to illustrate an example of how conventional power devices are arranged. FIG. 1 is a partial view of a power field effect transistor (FET) 10 that is made up of individual cells 11-13. Each of cells 11-13 is a transistor that has a source region, a drain region, and a gate structure, each cell 11-13 provides a portion of the current flow for FET 10. By electrically connecting each of the cells together, FET 10 has a current carrying capability that is approximately equal to the sum of each of the individual cells 11-13.

[0020] Applicants have discovered that with such conventional array configurations the power dissipation across the array of transistors is not uniform. When FET 10 is in operation, each of cells 11-13 generates heat that is proportional to the voltage potential times the current flowing through each cell 11-13. In addition, each cell 11-13 is further heated by the cells 11-13 that surround it. For example, a central cell 11 not only generates its own heat during operation, but is heated by adjacent cells 12, which in turn are heated by adjacent cells 13. The net effect is that cells in the center of an array are typically hotter than cells near the edge of the array.

[0021] This non-uniform power dissipation profile is illustrated in the graph of FIG. 2. The graph has an x-axis 16 that represents the position across a central portion of FET 10. The leftmost and rightmost portion of x-axis 16 represents the temperature increase in cells 13 along the periphery of FET 10. The central portion of x-axis 16 represents the increase in temperature of central cell 11. Y-axis 17 represents the increase in temperature of each cell. The change in temperature (

T) is the increase in temperature of a cell 11-13 as it transitions from a non-conducting to a conducting state in degrees Celsius (° C).

[0022] Arrows 18 represent the increase in temperature at periphery cells 13, arrows 19 represent the increase in temperature at cells 12, and arrow 20 represents the increase in temperature at central cell 11. As shown in the graph, central cell 11 is subjected to the highest temperatures during the operation of FET 10. The increase in temperature can exceed over 300° C., which exposes cell 11 to possibly fatal temperature conditions that destroy the structures that make up the transistor. Consequently, the amount of energy that FET 10 can dissipate is limited by the maximum temperature that cell 11 can be exposed to and still operate reliably.

[0023]FIG. 3 is a simplified representation of a semiconductor device 30 in accordance with the present invention. Preferably, semiconductor device 30 is a field effect transistor (FET) that has at least three terminals 32-34 that are connected to an array of transistors 31. As will be explained in more detail below, array of transistor 31 includes a repeating pattern of individual transistors that each have two current carrying electrodes, namely a source region and a drain region, and a control electrode that is used to control or enable a current flow between the current carrying electrodes.

[0024] Terminals 32-34 provide the necessary control signals and voltage potentials to array of transistors 31 to operate FET 30. Terminals 32-34 can be the leads, pins, solder balls, etc. that extend from a semiconductor package, or represent the bonding pads, bonding wires, or interconnect structures that are used internally to provide voltage potentials to array of transistors 31. Two terminals 32-33 are used to provide power supply voltages (e.g., VDD and ground potentials) and one terminal 34 is used to provide a control voltage that is used to control the flow of current through each transistor in array of transistors 31.

[0025] Turning now to FIG. 4, a more detailed description of array of transistors 31 is provided. Array of transistors 31 is divided into portions, sub-arrays of transistors, or sub-arrays 41-43 that are arranged on a semiconductor substrate 35. Each of sub-arrays 41-43 preferably comprises a plurality of field effect transistors that are configured in a repeating cell pattern so that each transistor is electrically connected together. Each of sub-arrays 41-43 is also electrically connected together so that all of the transistors cooperate to provide array of transistors 31.

[0026] Bonding pads 45-47 are carefully positioned throughout array of transistors 31 to provide heatsinks that more evenly distribute the energy dissipation across semiconductor substrate 35 when FET 30 is in operation. Bonding pads 45-47 are preferably connected to terminals 32-34 (see FIG. 3) with bonding wires or other interconnect structures so that bonding pads 45-47 can provide the necessary voltage potentials to array of transistors 31. For example, bonding pad 45 can provide the power supply voltage (i.e., VDD), bonding pad 46 can provide a ground potential, and bonding pad 47 can be used to provide the control voltage that enables the transistors in each of sub-arrays 41-43.

[0027] As shown in FIG. 4, sub-arrays 41-43 each have a different configuration, however, it should be understood that more than three configurations are possible. Sub-arrays 42 represent the transistors that make up the periphery cells of array of transistors 31. Sub-arrays 41 represent the transistors that are positioned centrally in array of transistors 31 between various bonding pads 45-47. Sub-arrays 41 represent the transistors that extend from between various bonding pads 45-47 to the periphery of array of transistors 31.

[0028] In contrast, conventional power FETs are made from one undivided array of transistors that position the bonding pads around the periphery of the array of transistors. However, the present invention divides the array of transistors up into sub-arrays of transistors 41-43 that are separated by bonding pads 45-47. Preferably, sub-arrays 41-43 contain the same number of transistors as a conventional device that has its bonding pads around the perimeter of the array. For example, bonding pad 45, which is connected to terminal 32 (see FIG. 3) is positioned between sub-array of field effect transistors 42 and second sub-array of field effect transistors 41. Bonding pad 46, which is connected to terminal 33 (see FIG. 3) is positioned between sub-array of field effect transistors 41 on two sides, and between sub-array of field effect transistors 43 and sub-array of field effect transistors 42 on its other sides.

[0029] As shown in FIG. 4, bonding pad 45 is separated from sub-array 42 by a distance (indicated with a line 50) that is preferably about 1 micrometer (μm) to 200 μm. Bonding pad 45 is also separated from sub-array 41 by a distance (indicated with a line 51) that is preferably about 1 μm to 200 μm. In the preferred embodiment, bonding pad 45 is equidistant from sub-array of field effect transistors 42 and sub-array of field effect transistors 41. Similarly, bonding pad 46 is equidistant from sub-arrays 4 1.

[0030] The reduction in the maximum temperature that any portion of array of transistor 31 is exposed to during operation can be further optimized by adjusting the width of bonding pads 45-47 (indicated with a line 54) relative to the width of sub-arrays 41 and 42 (indicated with lines 52 and lines 53, respectively). To properly operate as a heatsink for the energy created by the surrounding transistors, the width of bonding pads 45-47 should be within about thirty percent of the width of the sub-arrays 41. Preferably, the width of bonding pads 45-47 should be within about ten percent of the width of the sub-arrays 41, and even more preferably, the width of bonding pad 45 is approximately equal to the width of the sub-array 41. In addition the width of sub-array 42 should be greater than the width of the sub-array 41 because the transistors in sub-array of transistors 41 is surrounded by more active structures that generate heat when array of transistors 31 is in operation.

[0031] As shown in FIG. 4, sub-arrays with the same reference number are approximately equal in size. This is so the layout for array of transistors 31 can be as efficient as possible. It should be understood that each of sub-arrays 41-43 need not be the same size, and that each could even be different in size and shape.

[0032] Referring now to FIGS. 5 and 6, a more detailed explanation of the configuration of each sub-array of transistors 41-43 is provided. FIG. 5 is a top view of a portion of a sub-array 41-43, and FIG. 6 is a cross-sectional view taken along the section-lines 6-6 shown in FIG. 5. Each sub-array 41-43 is made up of individual field effect transistors 60-63 that are connected together to provide FET 30 (see FIG. 1). FIGS. 5-6 are intended to illustrate only a portion of a sub-array 41-43 as the structures shown in FIGS. 5-6 are preferably repeated across semiconductor substrate 35 to increase the current carrying capability of FET 30. Although FIG. 6 only shows four transistors 60-63, it should be understood that each sub-array 41-43 would probably have at least five transistors and could even have fifty or more transistors.

[0033] As shown in FIG. 6, each transistor 60-63 is made up of a gate structure 55, a drain region 66, and a source region 67. Metal lines 56 and 57, or other suitable interconnect material, are used to provide an electrical connection to drain regions 66 and source regions 67, respectively. Transistors 60-63 are configured so that adjacent transistors 60 and 61 share a common drain region 66, which are connected to other drain regions 66 and to bonding pads 45 (see FIG. 4). Transistors 60-63 also have common source regions 67, which are connected to other source regions 67 and to bonding pads 46. Each of gate structures 55 is electrically connected to the other gate structures 55. During the operation of FET 30, a control voltage is applied to gate structures 55 (e.g., from terminal 34 (see FIG. 3) or bonding pads 47 (see FIG. 4) to control the flow of current through transistors 60-63. The general direction of the current flow when FET 30 is in operation is indicated in FIG. 6 with arrows 64.

[0034] Transistors 60-63 are made using techniques well known in the art. Preferably, each of transistors 60-63 is formed so as to be identical to the other transistors in each sub-array 41-43, and thus, each of transistors 60-63 have a substantially identical gate length (indicated in FIG. 6 with a line 69) and a substantially identical threshold voltage.

[0035] The improvement in the distribution of energy dissipation that can be appreciated with the present invention is best illustrated in FIG. 7. FIG. 7 is a graph illustrating the increase in temperature across array of transistors 31 (see FIG. 4) that occurs when FET 30 is in operation. Along a y-axis 71 the change in temperature between when FET is in a non-conducting state versus when FET is in a conducting state is shown. An x-axis 70 represents various points across array of transistors 31. Arrows 72 and 74 indicates the increase in temperature that occurs in sub-array of transistors 42 and 41, respectively. An arrow 73 is used to illustrate the increase in temperature that occurs within bonding pad 45. As is apparent from the graph of FIG. 7, bonding pads 45-47 serve as heatsinks that help dissipate energy out of array of transistors 31.

[0036] It has been discovered that placing bonding pads throughout an array of transistors can reduce the maximum temperature that any one location within the array is exposed to by ten percent or more. Although on its face this improvement may seen insignificant, it has also been discovered that the lifetime of a device is exponentially proportionate to any decrease in the maximum temperature that the device is exposed to. Consequently, a ten percent reduction in maximum operating temperature doubles the lifetime of the device.

[0037] More importantly, the present invention can double the time to failure of a device without having to appreciably increase the size of the device. Instead of forming the bonding pads along the periphery of an array of transistors, the present invention distributes the bonding pads across the array while maintaining the same number of transistors in the array. Thus, the present invention only requires a nominal increase in the size of FET 30. Furthermore, the present invention does not require any additional processing steps. Therefore, the time to failure of a field effect transistor can be significantly improved without appreciably increasing its manufacturing cost.

[0038] Referring now to FIG. 8, an alternative configuration for sub-arrays of transistors 41-43 (see FIG. 4) is provided. Each of sub-arrays 41-43 can be made of a repeating pattern of vertical field effect transistors 88-90. As shown in FIG. 8, each transistor 88-90 is made up of a gate structure 81 and a source region 91. Transistors 88-91 are configured so that they all share a common drain region 86, which is connected to bonding pads 45 (see FIG. 4). Metal lines 84 and 83, or other suitable interconnect material, are used to provide an electrical connection to drain regions 86 and source regions 91, respectively. An additional doped region 85 may be used to provide electrical connection between metal line 84 and drain region 86.

[0039] Adjacent transistors 88 and 89 share a common source region 91, which is connected to other source regions 91 and to bonding pads 46. Each of gate structures 81 is electrically connected to the other gate structures 81. During operation, a control voltage is applied to gate structures 81 (e.g., from terminal 34 (see FIG. 3) or bonding pads 47 (see FIG. 4) to control the flow of current through transistors 88-90. The general direction of the current flow when FET 30 is in operation is indicated in FIG. 8 with arrows 87.

[0040] Transistors 88-89 are preferably formed simultaneously using techniques well known in the art so that source region 91 of each of transistors 88-90 is equally spaced apart from the source region 91 of adjacent transistors. The distance between adjacent source regions is indicated in FIG. 8 with a line 92 and is preferably about 0.5 microns to 10 microns.

[0041] Turning now to FIG. 9, an alternative configuration for an array of transistors 95 that can be used to provide FET 30 is provided. Rather than distribute heatsinks throughout an array of transistors 31 as shown in FIG. 4, this embodiment adjusts the energy dissipation profile of array of transistors 95 by altering the electrical characteristics of the individual transistors that make up array of transistors 95.

[0042] Array of transistors 95 is made up of field effect transistors that each have a source region, a drain region, and a gate structure 96. Each of the transistors is configured so that the source region of each transistor is electrically connected together, the drain region of each transistor is electrically connected together, and the gate structure 96 of each transistor is electrically connected together to provide FET 30. As shown in FIG. 9, electrical contact to the source region of each transistor is provided by an interconnect line 91, and electrical contact to the drain region of each transistor is provided by an interconnect line 92.

[0043] Dashed lines are used to divide array of transistors 95 into separate portions 101-105 that are used to explain how the energy dissipation of array of transistors 95 can be improved by adjusting the electrical performance characteristics of some of the transistors in array of transistors 95. Preferably, the physical geometries of all the transistors in each portion 101-105 of array of transistors 95 are identical because each transistor is formed simultaneously with the others. An additional ion implantation step can be used to adjust the threshold voltage of the transistors in portion 101 of array of array of transistors 95. For example, if the transistors in array of transistors 95 are n-channel enhancement mode devices, then doping portion 101 with a p-type dopant will increase the threshold voltage of the transistors in portion 101. Consequently, the transistors in portion 101 will conduct less current and dissipate less heat. Therefore, portion 101 of array of transistors 95 will have a smaller increase in temperature during operation compared to a device that formed the transistors in portion 101 with the same threshold voltage as the transistors in portions 102-105.

[0044] In the example shown in FIG. 9, array of transistors 95 was made up of lateral field effect transistors. FIGS. 10-11 are provided to illustrate that a similar reduction in the maximum operating temperature could also be achieved if an array of transistors 115 were made from vertical field effect transistors 111-112. As shown in FIG. 10, array of transistors is divided into a central portion 121 that is surrounded by outer or periphery portions 122-125. Array of transistors 115 is formed so that transistors 111 in portion 121 conduct less current and generate less heat than transistors 112 in portions 122-125. As a result, the central portion 121 of array of transistors has a lower operating temperature, and thus, a longer time to failure. Section lines 11-11 are used in FIG. 10 to indicate the location of the cross-section illustrated in FIG. 11.

[0045] Referring to FIG. 11, each of transistors 111-112 shares a common drain region 118 in semiconductor substrate 35 and has a gate structure 119 that is electrically connected to the other gate structures 119. Preferably, the material used to provide gate structures 119 is of sufficiently low resistance that the control voltage on each of transistors 111-112 is maintained at approximately the same voltage potential when FET 30 is in a conducting state. This is desirable so that each of transistors 111-112 in array of transistors 115 cooperate equally to provide FET 30. Transistors 111-112 also have a source region 114 that is electrically connected to the other source regions 114. Interconnect lines 113 provide electrical connection to source regions 114.

[0046] To reduce the amount of current flowing through the transistors in portion 121 of array of transistors 115, the pinch-off resistance of transistors 111 is increased by increasing the distance between adjacent source regions 114. For example, the distance between adjacent source regions 114 of transistors 111 (indicated in FIG. 11 with a line 117) is approximately 1 microns to 5 microns. In contrast, transistors 112 in portions 122 and 124 have source regions 114 that are further apart. This distance is indicated in FIG. 11 with a line 116 and this distance is about 5 μm to 10 μm. The structure can be achieved in a variety of ways such as by adjusting the ion implantation mask that is used to define the locations of source regions 114. Since the distance between adjacent source regions 114 in transistors 111 is less than in transistors 112, transistors 111 conduct less current than transistors 112. Therefore, transistors 111 generate less heat than transistors 112 so that the energy dissipation of portion 121 of array of transistors 115 is less than portions 122-125.

[0047] An alternative FET transistor structure that has thermal robustness uses a plurality of transistors connected collectively to form a single composite power transistor device for a given operating condition. In this form, a plurality of transistors each have the drains thereof connected together, the sources connected together and the gate connected together. In one form, the plurality of transistors are created with an array of transistors. Additionally, transistors in a thermal center or thermal central portion of the connections are a different device type than transistors in a peripheral portion of the connections. It should be understood that the thermal center of the composite FET structure may or may not be the geographic center of the composite FET structure. By different device type is meant that the electrical and thermal characteristics of the devices in the center are different than the electrical and thermal characteristics of the devices in the periphery. The electrical and thermal characteristics of the devices can be varied by controlling various components of the device, such as, drift length, channel length, field optimization layers (RESURF). In one form, increasing the channel length increases thermal breakdown. In another form, the electrical characteristic that is varied is the transistor drift length so that the device or devices in the center have a higher breakdown voltage than the device or devices in the periphery. In an LDMOS (Lateral Double Diffused Metal Oxide Semiconductor) field effect transistor (FET) device, the drift length is defined as a distance between a channel of the LDMOS device and the drain contact of the LDMOS device. It should be well understood that other types of devices (transistors or diodes) may be used, and that an LDMOS device is stated by way of example only. By having devices in the center of the collective power transistor structure with a higher breakdown voltage, the thermal profile of the field effect transistor is significantly enhanced. More current conducted by the field effect transistor is forced to the outside or peripheral transistors as a result of using two different types of transistors, for example differing drift lengths. The thermal central devices have a longer drift length and therefore the breakdown voltage is greater than the devices in the periphery. As a result, the transistors in the center permit higher voltage operation and permit higher temperature operation. Thermal failure of the central devices is therefore significantly reduced. By using transistors in the thermal center that have a higher thermal breakdown characteristic than in the periphery, a multiple-device-platform (MDP) based power driver has been provided. MDP refers to the fact that multiple types of devices are being used to form a power transistor or driver device. This embodiment provides a field effect transistor that can sustain more time and/or power before reaching a critical temperature that will cause damage to the device.

[0048]FIG. 12 graphically illustrates the improvement in thermal profile that a composite field effect transistor structure has using this embodiment. The vertical axis represents a maximum junction temperature of the field effect transistor and the horizontal axis represents the position within the active region of the device. As can be seen from the graph, higher temperature operation is assured. For example, maximum junction temperatures in the range of 450 degrees Centigrade and higher are achievable. In addition, higher temperature operation is assured over a larger portion of the entire structure or array.

[0049] An advantage of the embodiment that uses differing transistor drift lengths is the ability to use this embodiment easily with standard transistor libraries that have transistors of differing breakdown voltage characteristics. Breakdown power is known as the power value that causes any permanent damage to the material of the transistor. Many standard cell libraries classify transistor devices by their breakdown voltage capability. Breakdown voltages such as 25 volts, 45 volts, 65 volts, 85 volts and higher are examples. As a result, it is very easy to design a field effect transistor having a desired thermal profile by selecting thermal central positioned transistors from a library having a higher breakdown voltage than transistors positioned in the periphery. Devices with higher breakdown voltages conventionally have a higher on-resistance, Ron, than devices with lower breakdown voltages. This embodiment provides a resulting field effect transistor structure that has a slightly greater on-resistance than the cumulative Ron of only the peripheral transistors while significantly improving the energy capability of the resulting field effect transistor structure.

[0050] By now it should be appreciated that the present invention provides a method for changing the energy dissipation across an array of transistors. In one embodiment this is accomplished by distributing bonding pads throughout the array of transistors which serve as heatsinks when the transistors are in operation. Alternatively, the energy dissipation across an array of transistors can be adjusted by forming the transistors in a thermal central portion of the array of transistors so that they have a lower current density than transistors in the periphery of the array of transistors. For example the threshold voltage or pinch-off resistance of the transistors in the central portion of the array of transistors can be increased. Alternatively, the energy dissipation across a plurality of transistors can be adjusted by forming the transistors in a central portion of the plurality so that they have higher thermal breakdown characteristics than the transistors around the periphery. The device with higher thermal breakdown characteristics may be placed for MOS transistors or other types of transistors in the thermal central portion. By way of example, a change in transistor drift length may be sufficient to adjust the energy dissipation capability. In another example, both a change in transistor drift length and an optimization of RESURF layers can be made to adjust the energy dissipation capability.

[0051] A field effect transistor in accordance with the present invention can be made from the same number of transistors and in the same amount of space as conventional devices, yet have a significant improvement in both its power dissipation profile and its time to failure. Furthermore, the benefits of the present invention are not limited to power devices that have relatively short “on” times as the boding pads placed throughout the array of transistors have sufficient heatsink capability to allow the FET to be operated for “on” times in excess of 10 micro-seconds (ms). 

1. A composite field effect transistor comprising: a plurality of transistors each having a first current electrode thereof connected together, a second current electrode thereof connected together, and a control electrode thereof connected together, the plurality of transistors being arranged to form one or more transistors in a thermal central portion and a remainder in a thermal peripheral portion, wherein the one or more transistors in the thermal central portion have a higher thermal breakdown characteristic than the remainder in the thermal peripheral portion.
 2. The composite field effect transistor of claim 1 wherein the plurality of transistors are MOS transistors and the higher thermal breakdown characteristic of the one or more transistors in the thermal central portion is provided by using a longer drift length than used for the remainder in the peripheral portion.
 3. The composite field effect transistor of claim 1 wherein the plurality of transistors are formed in an array.
 4. The composite field effect transistor of claim 1 wherein the higher thermal breakdown characteristic is a breakdown voltage characteristic.
 5. A composite power transistor comprising: a plurality of transistor devices arranged to have transistors in a thermal central portion and in a thermal peripheral portion such that the transistors in the central portion have higher thermal breakdown characteristics than in the thermal peripheral portion.
 6. The composite power transistor of claim 5 wherein the plurality of transistor devices are LDMOS transistors and have differing drift lengths to create the higher thermal breakdown characteristics.
 7. The composite power transistor of claim 5 wherein the plurality of transistor devices have different breakdown voltages to create the higher thermal breakdown characteristics.
 8. The composite power transistor of claim 5 wherein the transistors in the thermal central portion have higher thermal breakdown characteristics by adjusting field optimization layers of the transistors in the thermal central portion.
 9. The composite power transistor of claim 5 wherein the transistors in the thermal central portion have higher thermal breakdown characteristics by adjusting both drift lengths and field optimization layers of the transistors in the thermal central portion.
 10. An array of transistors comprising a plurality of MOS transistors each having a first current electrode thereof connected together, a second current electrode thereof connected together, and a control electrode thereof connected together, the plurality of MOS transistors being arranged to form a first sub-array of transistors in a central portion and a second sub-array in a peripheral portion, wherein the first sub-array of transistors in the central portion have a higher thermal breakdown characteristic than the remainder in the peripheral portion.
 11. The array of transistors of claim 10 wherein the higher thermal breakdown characteristic is created by increasing drift length of the first sub-array of transistors in the central portion. 